uscan: redirect alioth gemwatch to gemwatch.debian.net

The Ruby team used to run a uscan redirector at https://pkg-ruby-extras.alioth.debian.org/cgi-bin/gemwatch

It is not simply a redirector; it also converts Ruby .gem packages to tarballs for Debian packaging, so it can't just be folded into e.g. the qa.debian.org redirector.

This service was migrated to https://gemwatch.debian.net/

Right now the original CGI script was turned into a redirector to the new service, and new packages produced by dh-make-ruby will already point to it. However, we still have 900+ debian/watch files pointing to the old URL, and the redirector will cease to work soon when alioth goes away. The existing references will be migrated, but it is not feasible to complete this migration process before the alioth shutdown.

This is a mitigation measure. It can and should be removed when the migration is finished.